As a substrate to form a semiconductor device to increase the hole mobility, a substrate having a strained silicon layer has received a great deal of attention. A layer (SiGe layer) made of silicon (Si) and germanium (Ge) is grown on a silicon substrate, and a single-crystal silicon layer is grown on that layer. Accordingly, a strain is applied to the silicon layer, and a strained silicon layer is obtained. This strain occurs because the lattice constant of the SiGe layer is slightly larger than that of the single-crystal silicon layer.
Tayanaka has reported a semiconductor substrate which facilitates separation of a thin semiconductor layer by forming a layer containing SiGe, GaAs, GaP, or GaN on a silicon substrate, forming a porous layer by anodizing, and forming a thin semiconductor film on the porous layer, and a manufacturing method of the semiconductor substrate (Japanese Patent Laid-Open No. 11-195562).
As a characteristic feature of the above-described strained silicon manufacturing technique, a relaxed SiGe layer is formed on a substrate. To relax the SiGe layer, crystal defects on the interface to the substrate are used. Since the defects propagate to even the uppermost silicon layer, it is difficult to obtain a high-quality silicon layer. In addition, to relax the SiGe layer, a thick layer of several μm is necessary. Hence, the manufacturing cost is high.
As a characteristic feature of the technique of Tayanaka, a porous layer containing SiGe, GaAs, GaP, or GaN is used as the separation layer of the thin silicon layer. This is different from the viewpoint of the present invention, i.e., using a porous layer as a strain induction layer.